This invention relates to an emitter-coupled multivibrator circuit comprising an oscillator circuit having two switching transistors, a current control circuit and a voltage supply circuit for supplying at least one reference voltage.
Such circuits of a frequency-controllable oscillator, which are essentially based on the operating principle cf the emitter-coupled multivibrator and are used in the field cf bipolar integrated circuits, for example, as a voltage or current controlled oscillator (VC0 or CC0) and as a fixed oscillator in phase control circuits or as a frequency modulator, are known from German Patents P 2540867 C2 and P 3632458 C2 which corresponds to U.S. Pat. Nos. 3,904,989 and 4,717,892, respectively.
An object of these known applications is to provide oscillation frequencies that are independent of supply voltage and free from drift. For the desired freedom from drift, it is essentially required that the charge current of a capacitor and the upper and a lower threshold voltage, upon the occurrence of which the oscillator triggers, be independent of temperature, while their dependencies compensate each other.
The said patents describe ways of reducing the frequency drift of the emitter-coupled multivibratcr. However, the two known techniques have the disadvantage that either they are not particularly suitable for the mass production of integrated circuits or they require extremely complex networks.
For example, DE PS 2540867 C2 discloses a temperature-compensated emitter-coupled multivibrator circuit having two switching transistors, a time control capacitor between the emitter electrodes of these switching transistors and two current sources connected between the emitter electrodes of the switching transistors and a fixed potential. The supplied current is controlled by a reference voltage, while a complicated device for controlling the collector currents of the switching transistors is provided in such a manner that the multivibratcr triggers when the voltage across the time control capacitor is substantially equal to the depletion layer voltage of the semiconductor junction at a current flow which is equal to the predetermined multiple of a current supplied by the first and second current sources. This circuit moreover has a further complicated device for producing a reference voltage for the current sources such that the reference voltage is substantially equal to the depletion layer voltage of the semiconductor junction. Such a principle circuit is already extremely complex. However, it becomes even more complicated when a voltage control for linearly varying the frequency of the multivibrator under the same or more favorable temperature conditions is desired. A corresponding circuit then aims at a reduction of the temperature-dependent base current variations, which occur because of temperature-dependent variations of the transistor current amplifications. As suitable principles two methods are used, i e. the reduction of the transistor base current by the use of Darlington transistors and the addition of circuits for deriving or supplying a temperature-dependent current for base current compensation, i.e. a compensation current which is substantially equal to the troublesome temperature-dependent base current. Such a circuit is then so complex that, although it is still suitable for mass production of integrated circuits, it is so very complicated that disturbances are to be expected.